This invention relates generally to the measurement of transparent elements in an opaque medium and particularly to the measurement of the transparent dots in a kinescope faceplate panel, or shadow mask.
During the production of picture tubes for color television receivers, a black matrix material is applied to the inside surface of the faceplate panels. The black matrix coating includes a large number of transparent dots which are arranged in a particular pattern. The transparent dots are coated with slurries of photosensitive materials including phosphors which emit the three primary colors of light when impacted by electrons. The phosphors are alternately applied in a repetitive sequence such as red green blue, to the transparent dots. Prior to the application of the phosphors, it is desirable to measure the widths of the transparent dots, and the opaque matrix spaces between the dots, to verify that they are within acceptable dimensional tolerances to avoid the expensive application of phosphors to improperly matrixed panels.
In order to measure the transparent dots, and the opaque spaces between the dots, the faceplate panel is placed between a stationary light source and a detector enabling light to pass through the dots to the detector. Portions of the panel are scanned with light and variations caused by the opaque spaces and the transparent dots are detected and provided to a measuring system.
Typically, the detector is a photosensitive solid state device such as a linear photodiode array or a CCD. The pixels of the array are charged to various levels in accordance with the amount of light impacting the individual pixels. A camera is used to focus the matrix dot pattern onto the array. The camera lens focuses and magnifies the image so that each dot spans a large number of detector pixels. Scanning is affected by rotating a galvo to change the portion of the dot pattern which is focused onto the array. The pixel charge levels are periodically clocked out to effectively scan a large number of lines across the dot pattern. The dots are aligned in rows and scanning is parallel to the rows. Accordingly some scanning occurs between rows of dots and no useful data is provided for such scans. The analog signals from the photodiode array are digitized and provided to a storage device within the measuring system. Data from all the scan lines must be provided to the storage device and, accordingly, the unusable data obtained from scanning between the rows of dots are also provided to the storage. For this reason there is a need for a method for rapidly identifying unuseful data to avoid processing such data and to speed up the processing of useful data. The present invention fulfills this need.